This invention relates generally to sockets for receiving packaged integrated circuits for test purposes, and more particularly the invention relates to a test socket for a dual in-line semiconductor package, or DIP.
In the manufacture of integrated semiconductor circuits (ICs), the final packaged IC must be subjected to testing. Sockets must be provided for receiving and protecting leads of the IC during the test. Typically a plurality of test sockets are mounted on a printed circuit board (PCB) with a PCB providing interconnections between the ICs and test equipment.
A conventional test socket is shown in an exploded perspective view in FIG. 1. Pins 10 having holes for receiving the IC leads are housed between a lower plate 12 and an upper plate 14. The diameter of each pin 10 is smaller at the bottom for reception in a hole 16 through bottom plate 12 with a flange on the upper end of each pin 10 being received in a recessed larger portion of each hole. Once the pins are assembled in bottom plate 12, top plate 14 is assembled to bottom plate 12 by suitable fasteners such as screws (not shown) to retain the pins in the bottom plate. Top plate 14 has holes 18 extending therethrough in alignment with pins 10 and which receive leads of an IC package. However, holes 18 are smaller in diameter than the flanges of pins 10 whereby the pins are captured between the upper and lower plates. FIG. 2 is a side view in section of the assembled socket taken along a row of pins.
In assembling the test sockets on a PCB, the pins of the socket are aligned with conductive pads on the PCB. It is essential that the connection between the socket pins and the pads that serve as a mounting point to the conductive traces on the PCB be of high quality. To this end, prior art sockets use gold paste to attach the socket pins to the PCB pads. The sockets are mounted to the PCB using screws after a small amount of gold paste is applied between the pins and the pads, and the assembly is then cured at a certain high temperature to achieve a good bond between the pins and the pads. The completed structure as shown in section view in FIG. 3.
Unfortunately, there are several problems with this structure and process. One problem is due to a certain degree of variation in the height of the pins that is a result of the imperfect nature of any pin manufacturing process. If a pin is short enough, then the gold paste will not be able to fill the gap between the pin and the underlying pad, thus creating an open circuit. Further, if too much gold paste is applied between pins and pads, the gold paste may overrun into an adjacent pin/pad, thereby creating an undesired short circuit. These problems are illustrated in the section view of FIG. 4. Exacerbating the situation is the fact that the gold paste method is an irreversible process, there is no simple process to return the gold paste back to original liquid state once securing is complete. Thus, if an open or short circuit problem is discovered, it is very difficult to make repairs to the affected sites.
The present invention overcomes these problems with prior art test sockets.
In accordance with the invention, electrically conductive springs are used to make electrical contact between connector pins of a test socket and contact pads on a PCB on which the socket is mounted. Accordingly, the springs can compensate for different lengths of connector pins.
In a preferred embodiment, a spring holder plate is positioned on a PCB with the holder plate having holes in alignment with contact pads on the PCB, the holes having a diameter for accepting springs whose diameters are less than the diameters of the contact pads.
Springs made of high temperature spring wire are placed in the holes in the holder plate, and then the test socket is mounted on the holder plate with socket pins aligned with and engaging the springs. The socket can comprise a bottom plate and a top plate, similar to the prior art. Screws or other fasteners then attach the socket to the holder plate or PCB or both.
The length of the springs compensates for differences in lengths of the socket contacts. The holes in the holder plate prevent the springs from contacting each other and shorting. Furthermore, if there are any defective springs or contacts in the assembly, the structure can be readily disassembled for repairs.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.